In recent years increasing attention is given to the exploration and utilisation of natural gas resources around the globe. A disadvantage of natural gas with respect to oil is the difficulty to transport large volumes of natural gas from the source to the market. One way of efficiently transporting natural gas is by liquefying the natural gas and to transport the liquefied natural gas (LNG). Another way is to convert the methane in the natural gas to liquid hydrocarbons using a Gas-to-Liquid process (GtL). The GtL products are typically liquid and can be transported in a similar way as traditional oil and oil products.
Besides methane, the natural gas typically comprises other hydrocarbons such as ethane, propane, and butanes. Such a natural gas is referred to as wet gas. The latter two can be added to the LPG pool, however, ethane cannot. Moreover, for various reasons the ethane content in the natural gas supplied to an LNG or GtL process is restricted and therefore a significant part of the ethane must be removed from the natural gas prior to providing the natural gas to either a LNG or GtL process.
Although, the application of ethane is limited, typically ethane is combusted in a furnace to provide heat; its corresponding olefin ethylene is a base chemical with a wide application and is of great commercial interest. Ethane can be converted into ethylene, e.g. using a thermal cracking process. Subsequently, the ethylene can be used to produce e.g. polyethylene, styrene or mono-ethylene-glycol. The conversion of ethane to ethylene is highly endothermic and requires significant energy input. In addition, the capex for the ethane to ethylene process, in particular the back-end work-up section, and the subsequent ethylene conversion processes is high and a minimum ethylene production capacity is required to make it economically benign.
When, the ethane content in the natural gas is too low, and consequently insufficient ethane is available, the ethane/ethylene route becomes unattractive.
This problem becomes even more pronounced, in the case where the natural gas is withdrawn from relatively small reservoirs, especially those located in remote isolate locations, also revered to as stranded natural gas. Of course, this stranded natural gas may be converted to LNG or GtL products. However, this requires the stranded gas reservoir to sustain a minimum production level per day in order to make the investments worthwhile. Typically, such stranded natural gas reservoirs cannot achieve sufficient production levels to sustain a GtL or LNG plant. In addition, insufficient ethane is co-produced to sustain an ethane to ethylene process and subsequent ethylene conversion processes.
It has been suggested to combine an ethane steam cracker with Oxygenate-to-Olefin (OTO) processes, which can produce additional ethylene. For instance, C. Eng et al. (C. Eng, E. Arnold, E Vora, T. Fuglerud, S. Kvisle, H. Nilsen, Integration of the UOP/HYDRO MTO Process into Ethylene plants, 10th Ethylene Producers' Conference, New Orleans, USA, 1998) have suggested to combine UOP's Methanol-to-Olefins (MTO) process with a naphtha or ethane fed steam cracker. It is mentioned that by combining both processes sufficient ethylene can be produced, while coproducing valuable propylene. A disadvantage mentioned by C. Eng et al. is the fluctuating price of methanol, which is the primary feed to the MTO reaction.
In WO 2009/039948 A2, a combined stream cracking and OTO process is suggested for preparing ethylene and propylene. According to WO 2009/039948 A2, in this process, a particular advantage is obtained by combing the back-end of both processes. The methanol feedstock is produced from methane, requiring a sufficient supply of methane.
In US2005/0038304, an integrated system for producing ethylene and propylene from an OTO system and a steam cracking system is disclosed. According to US2005/0038304, in this process, a particular advantage is obtained by combining the back-end of both processes. The methanol feedstock to the OTO process is produced from synthesis gas. However, according to US2005/0038304 the production of methanol from synthesis gas has high energy requirements due to the endothermic nature of the synthesis gas production process, such an endothermic synthesis gas production process is normally steam methane reforming.
In US2002/0143220A1, a process is described for producing olefins. A hydrocarbon feedstock is oxidatively dehydrogenated to produced olefins and synthesis gas. The synthesis gas may be converted to methanol. The methanol may be converted to ethylene.
A problem with using natural gas to produce ethylene is that together with the hydrocarbons in the natural gas a substantial amount of carbon dioxide may be co-produced from the subsurface natural gas or oil reservoir. Such carbon dioxide is also referred to as field carbon dioxide. Some subsurface natural gas or oil reservoirs comprise substantial concentrations of carbon dioxide, up to 70 mol % based on the total content of the gas extracted from the reservoir. This carbon dioxide must be sequestrated or otherwise captured and stored.
An additional problem is that, especially when the carbon dioxide content is high, not enough ethane is produced from the subsurface reservoir to sustain a ethane cracker with full work-up section.
In US2007/049647A1 it is described that carbon dioxide co-produced with natural gas may be mixed with synthesis gas to produce a methanol feed for a methanol-to-olefins process.
In WO2007/142739A2, a process is described for producing methanol from synthesis gas. The methanol may be used for producing olefins. In the process described in WO2007/142739A2, a hydrogen stream comprising greater than 5 mol % methane is combined with the synthesis gas. The hydrogen stream may for instance be obtained from a steam cracking process. According to WO2007/142739A2 it is desirable that the amount of CO2 in the synthesis gas feedstock is minimized to reduce the need for later separation of water from the crude methanol.
There is a need in the art for a process for producing ethylene from a feed comprising, besides hydrocarbons, carbon dioxide, wherein the amount of carbon dioxide that needs to be sequestrated or otherwise captured and stored is further reduced.